Two-way telecommunication systems are often prone to the phenomenon of echo. Echo is a result of coupling between signals travelling in opposite directions within a telecommunication system. For example, reflections from a four-wire to two-wire hybrid are a well known source of echo in telephone networks. Other sources of echo include acoustic or electromagnetic coupling of received signals and transmitted signals within a terminal device, such as a telephone. Acoustic echo, for example, can result from the microphone of a telephone responding to audio signals emitted by the speaker. Acoustic echo can be particularly acute in an enclosed environment, such as in mobile cellular telephony.
Echo cancelers are commonly employed to substantially reduce transmission of echoes within telecommunication networks. A traditional echo canceler includes a transversal filter which is characterized by coefficients of the impulse response of the near-end terminal equipment. The transversal filter is connected to receive a near-end receive signal, and to produce an estimated echo signal. The estimated echo signal is subtracted from the send signal transmitted by the near-end terminal equipment. Echo cancelers have contributed substantially to the intelligibility of low-loss long distance, satellite and fiber optic telephone networks. Examples of such echo cancelers are described in U.S. Pat. Nos. 4,064,379; 4,113,997; 4,321,686; 4,360,712; 4,377,793; and 4,600,815 to Horna, and in U.S. Pat. Nos. 3,780,233; 3,789,233; 3,836,734; 3,894,200 to Campanella.
Even the best echo cancelers are imperfect at entirely removing echo, due to the finite amount of time required to initially develop, and to subsequently update, the presumed impulse response of the near-end echo path. Additionally, the inherent quantization error of digital signals imposes a limit on the accuracy with which echo can be estimated and removed. Thus, a residual echo component, that is distinct from uncorrelated background noise, remains within an echo-canceled signal. The residual echo component is presumed to have a low amplitude. Hence, the traditional approach to reducing residual echo has been to employ a center clipper for blocking the low amplitude components of echo-canceled signals. Typically, the center clipper is selectively deactivated when the near-end send signal is large, in order to avoid zero-crossing distortion of transmitted speech. However, selective activation and de-activation of the center clipper also tends to undesirably modulate low-amplitude background noise components of the transmitted signal. Such modulation can be perceived at the far end as occasional disconnection of the communication channel. Additionally, selectively-activated center clippers can undesirably clip softly-spoken syllables.
U.S. Pat. Nos. 5,157,653 and 5,283,784 to Genter, describe an echo canceler wherein a variable attenuator is employed to attenuate the echo-canceled signal over a continuous range of attenuation. The attenuation is determined by an attenuation factor that is varied in response to comparison of relative levels of the receive signal, the send signal, and the echo-canceled signal, in order to attenuate residual echo. To reduce the perceptibility of the operation of the attenuator, changes in the attenuation factor are made gradually during finite intervals of time, rather than abruptly.
To further reduce theperceptibility of variable attenuation for residual echo reduction, the aforementioned Genter patent discloses injection of a compensating noise component into the attenuated echo-canceled signal. The level of the injected noise component is determined on the basis of an estimated noise component of the send signal and the prevailing attenuation applied by the attenuator. As further described therein, the compensating noise component is derived from a noise source for providing a noise signal having a predetermined spectral characteristic, such as white noise or noise that is otherwise band shaped to approximate typical telephone circuit noise. Of course, "typical" telephone circuit noise can vary considerably, as can ambient acoustic noise in the vicinity of a telephone user. For example, telephone signals produced by a mobile user of a cellular radio telephone system may include noise that is spectrally distinct from noise within telephone signals produced by a stationary user of a twisted-pair local loop. Hence, even a slow transition from the actual background noise to an artificial injected noise may be distinctly perceptible and distracting to the far end listener. Thus, it would be desirable to provide an echo canceler wherein an injected noise component is more accurately related to the prevailing noise spectrum within the transmitted signal.